54 JoLY — An Estimate of the Geological Age of the Earth. 



VII. — Uniformity of Denudation by Solution. 



Land area and Rainfall. — The most prominent considerations involved in the 

 question of how far the present rate of denudation by solution may be accepted 

 as an average of that extending over past times are that of the varying ratios of 

 land and sea areas of the past and the amount of rainfall received upon the latter. 

 The fact that palaeontologically similar deposits in the various parts of the 

 world are not necessarily contemporaneous, but homotaxial, debars the geologist 

 from mapping the sediments of any horizon (even were tliese fully known) as 

 forming part simultaneously of the oceanic area. Could he even claim full 

 assurance here, the land areas supplying the sediments must still remain unknown. 



In this difficulty indirect inferences only can be resorted to. 



Those who accept the stability of the continents and oceans as a whole cannot well 

 admit that the balance of land and water was ever very seriously interfered with. 

 Sir J. Murray* has calculated that if the present land of the globe were I'cduced 

 to the sea level by being removed to and piled up in the shallow waters of 

 the ocean, its extent would be altered from the present 55 x 10^ to 80 x 10^ 

 square miles, the ocean simultaneously changing from 137'2 x 10'' to 113 x 10^ 

 square miles. The mean height of the land, which is at present 2250 feet, would 

 become ; while the mean depth of the ocean, at present 2080 fathoms, would 

 increase to 3 miles, 23-45 x 10^ cubic miles of material being transported into the 

 sea. 



If the Earth's crust were rigid, and neither subsidence or elevation ever took 

 place, such a calculation would mark the extreme distribution of the existing 

 sub-aerial material which would be possible under the action of denuding agencies. 

 It could only be brought about by an infinitely prolonged denudation and 

 quiescence of the crust. 



As a matter of fact, however, we know that over the continental areas there 

 have been frequent depressions and elevations, and these acting alternately again 

 and again over the same area. The Uniformitarian, we assume, regards this 

 shifting balance of land and water as confined mainly to the area indicated above, 

 the 80 million square miles marking out the elevated plateaux of the globe. The 

 dry land of to-day occupies some 68 per cent, of this area. It cannot be 

 supposed to have ever occupied 100 per cent, of it, for then sediments must have 

 been laid down in the present ocean troughs. That such sedimentation, again, as 



the glass, and the taking up of a small additional amount of potash and soda (apparently from the sea) — is 

 hardly sufficiently abundant, according to present knowledge, to j ustify consideration, here. See the Report 

 on the Deposits, p. 304. The Phillipsite appears to he a purely alteration product of the basic (?e'ir«s. See 

 Merrill {loc. cit., p. 375). 



* Scottish Geological Magazine, 1888, pp. 1 et seq. 



